This invention relates to a method of ultrasonic flaw detection of a pipe to detect flaws in a base material and a weld of the pipe by transmitting ultrasonic waves therethrough. The method is applicable to reactor pipes at a chemical plant such as a hydrogen manufacturing reformer tube, and to other pipes used for varied functions.
The method of ultrasonic flaw detection of a pipe according to the prior art, generally, is explained with reference to FIG. 1 of the present invention. As seen, an incident sound from a transmitting probe 3a mounted on a pipe adjacent a test piece part 2A (which is a weld 2 in this example) is received by a receiving probe 3b mounted on the pipe adjacent and across the test piece part 2A, and whether there is a flaw in the test piece part 2A or not is determined by attenuation of the sound occurring as it traverses the test piece part 2A. With such a flaw detection method, however, the attenuation of the sound is affected by macrostructures, surface roughness and the like of the pipe, and is received in an affected state by the receiving probe 3b to measure its decibel value, which results in a very low degree of detecting precision.
In order to eliminate the above disadvantage, a method as explained with reference to FIGS. 2 and 3 of the invention has been proposed, in which the decibel values of through-transmission sounds as received are measured using the transmitting probe 3a and the receiving probe 3b at two pipe positions opposite to each other relative to the test piece part 2A, and an arithmetic mean of these decibel values is compared with a decibel value of a through-transmission sound obtained at the test piece part 2A, a difference therebetween providing a basis for determining presence of a flaw. This method is effective to offset the adverse influence of marcrostructures, surface roughness and the like of the pipe and to detect flaws with a relatively high precision. However, according to this method, the arithmetic mean of the decibel values of the received sounds measured at two positions adjacent the test piece part 2A (i.e. only one position at each side of the test piece part 2A) is regarded as the mean value throughout an entire periphery of the pipe. Therefore, this method after all lacks in detecting precision when applied to pipes whose macrostructures and surface roughness are uneven in the peripheral direction.
To be particular, an austenic heat resisting cast steel pipe to be used as a steam catalytic reforming heater pipe, for example, which is usually made by centrifugal casting has macrostructures distributed quite unevenly not only in the axial direction but in the peripheral direction also. Since the foregoing method carries out flaw detection without regard to such a peripheral distribution of macrostructures, it is impossible to detect flaws with a truly good precision or to provide a quantitative indication of tendencies of the flaws.